Such devices are, for example, commonly used by those with diabetes for the administration of insulin.
In such devices the displaceable piston is driven to cause medicament to be expelled by way of a lead screw or other piston means. The lead screw may conveniently be advanced and retracted by way of a suitable drive mechanism. It is known to provide a drive mechanism in which the rotary movement of a motor or other suitable drive means is transformed into linear movement of the lead screw.
It is also known to provide a guideway or guide housing through which the lead screw travels. However, in known devices the guideway is a separate component which is held in the desired operating position by other components of the apparatus and which increases the overall length of the medicament delivery device.
For example, in EP 0 143 895 there is disclosed an infusion device which comprises a housing having a shaped cavity formed therein. A first part of the cavity serves as a mounting support for a replaceable medicament cartridge. A rotatable driver sleeve is located in a second part of the cavity, which acts as a thrust bearing for the rotatable driver. The lead screw is adapted to travel through the driver sleeve. The threaded lead screw has opposing flat surfaces. The inside of the driver sleeve conforms to the shape of the lead screw to prevent rotation of the lead screw independently of the driver sleeve. The guideway takes the form of a threaded nut 56 threaded onto the lead screw. The nut is located in a specially shaped locating recess of the housing cavity to prevent rotation of the nut. Axial movement of the nut is prevented because in use, the nut is trapped between on one side the housing and on the other by one end of the medicament cartridge cylinder. In use, the driver sleeve carries along the threaded rod that moves ahead as a screw spindle through the nut or guideway held in the housing.
Such devices are complex to manufacture. The shaped cavity requires a suitable tolerance both correctly to receive the driver sleeve and to provide a suitably shaped locating recess for the nut acting as a guideway. In addition, correct location of the cartridge cylinder is required to prevent the nut from becoming loose and the correct location of the nut must be ensured every tune that a medicament cartridge is replaced. Such disassembly and reassembly when a cartridge is to be replaced is a disadvantage if the user, as is common amongst those with diabetes, is infirm or has impaired vision.
There is a need for a simpler and more compact arrangement for a medicament delivery device, which arrangement removes or at least reduces the above mentioned problems and which, in particular, can be used in a medicament delivery device without such an shaped cavity as described in the prior art explained above.